There are known systems for the wireless transmission of signals, utilized, for example, in numerical control machine tools for transmitting signals indicative of the position and/or dimension of mechanical pieces, generated by checking heads or contact detecting probes mounted in the machine. More specifically, in the course of checking cycles, the probes displace with respect to the piece to be checked, touch piece surfaces and respond to contact by generating a contact signal. The contact signal is processed by processing devices for enabling suitable transmission devices to wirelessly transmit signals representative of contact to receiver units. In turn each receiver unit is connected, by means of an interface device, to its associated numerical control unit. By processing other signals relating to the mutual spatial position between probe and piece, the numerical control unit obtains information about the position of the piece surfaces. The transmitted signals can be, for example, electromagnetic signals of optical or radio-frequency type. Typically probes are power supplied by batteries, located right at the interior of the probes. In order to preserve battery life, so avoiding possible operation defects and too frequent substitutions, it is necessary to limit as far as possible power consumption.
In order to accurately identify the contact point between the probe and the piece, it is required that the delays—necessarily introduced when transmitting the state of the probe—be sufficiently short and, above all, accurate and repeatable.
There are known in the art systems and methods that enable to obtain an extremely accurate and repeatable introduced delay.
For example, it is known to activate a pulse generator, or “clock” generator, at the moment when contact between the probe and the piece occurs and keep it activated in the course of the checking, in order to generate a reference signal with highly stable frequency, the cycles of which are counted by a counter, while probe logic circuits perform the required checking operations. The total number of cycles of the reference signal (or, in an equivalent way, of the clock) counted by the counter are preset in a manner so that the operations that the probe logic circuits perform in the course of the checking (for example, operations relating to the transmission of a previous contact signal, or signals representative of the state of the probe) are always completed before the count ends. At the end of the count, an output signal indicative of contact is transmitted to receiver units.
FIGS. 1 and 2 show, in simplified form, what mentioned above in case of two different time intervals, Δt1CPU and Δt2CPU, respectively, necessary for allowing the probe logic circuits to complete the required checking operations. More specifically, the figures show the trend—as a function of time t in the time interval between an instant t0 (moment when contact occurs) and an instant t1 (moment of transmission of the output signal)—of a reference signal RS, of a signal CPUA representative of the operations carried out by the logic circuits after contact, of an output signal OS01 and of a contact signal TS01. It can be realized that the output signal OS01 of both FIGS. 1 and 2, is generated and transmitted at the instant t1 after an identical delay Δt01 starting from the instant t0 regardless of the time interval Δt1CPU and Δt2CPU of the signal CPUA, i.e. the duration of the operations performed by the probe logic circuits in the course of the checking. According to this known method, the accuracy of the delay Δt01 generated between contact and transmission of the associated signal is strictly correlated to the stability of the frequency of the reference signal and to the short activation (or “start-up”) time of the clock, defined as the time necessary for the clock to activate further to an edge of the contact signal, or other suitable signal. In order to have a reference signal which is stable in frequency for a long time interval with respect to the period of the signal, it is necessary to implement the clock by means of, for example, a quartz crystal resonator or similar devices. However, these devices have variable and long start-up times, in the order of tens of thousands of times the period of the generated signals. In practice, there are not known in the art devices, as oscillators or resonators, that concurrently provide stable frequencies for relatively long time intervals and relatively short start-up times. Furthermore, it is known that the energy consumption of an oscillator increases as its oscillation frequency increases.
A different method, substantially alike the method mentioned at the beginning of the description, foresees the activation of the crystal resonator at the start of the checking (before contact occurs), while just the counter is activated at the edge of the contact signal.
In this case the selection of the frequency of the reference signal is critical because the oscillation period defines the resolution that would be obtained in the amount of time delay between the instant of contact and the start of the transmission of the output signal.
In fact, the counter cannot activate at any whatever moment, but must wait for a change of state, in other words an edge, typically the rising edge, of the reference signal.
The consequence of the above can be readily seen in FIG. 3 where a relatively slow clock (that generates a reference signal RS with a relatively long period TRS) is shown and three contact signals TSA, TSB, TSC are represented at three different moments of contact t0A, t0B, t0C; for all the signals TSA, TSB, TSC the count always starts at a time tC at the same first rising edge of the reference signal RS subsequent to contact, so that their original time separation no longer exists and an output signal OSABC is generated and transmitted for all signals at the same time tABC, exactly after an identical delay Δt from the instant tC.
In order to overcome this kind of problem, European patent application EP-A-0826201 suggests to slightly alter (to increase or to decrease) the period of the output signal representative of contact, so as to keep the delay between contact and the end of the transmission of the output signal constant.
Another manner to overcome the above mentioned problem foresees to increase the resolution of the delay time, by increasing the frequency of the reference signal.
In the previously described applications, a typically requested resolution is in the order of 1 μs, that corresponds to a minimum frequency of the reference signal of 1 MHz. Operating at similar frequencies is quite problematic in a battery-powered system, owing to the high and constant consumption of current of the associated clock.
Therefore, in the checking systems equipped with battery-powered contact detecting probes, it would be expedient to utilize an oscillator/resonator that guarantees a short start-up time and a reference signal with stable frequency in time, in order to assure transmission with good repeatability characteristics. Furthermore, there is the requirement of low energy consumption to extend the battery life.
Unfortunately, as hereinbefore stated, it is practically impossible to have all these characteristics in a single oscillator/resonator and it is necessary to accept, often unsatisfactory, compromise solutions in the existing known systems and methods.